This invention relates to the monitoring of atmospheric pollutants and more particularly to the monitoring of the mass concentration of such pollutants above and below a given size.
Air borne particulates, or atmospheric pollutants, are of continuing concern because of their effects on human health, climatic conditions and visibility in many urban areas. Present air quality standards for these pollutants are expressed in terms of total aerosol mass. However, such a single parameter measurement may be inadequate, if not misleading, because it has been found that the pollutants being monitored have two size ranges, which may be referred to as coarse and fine, and that the pollutants in one size range are produced by different sources and are associated with different effects than pollutants in the other size range. The coarse or larger pollutants are typically 5 microns or greater in size and are composed of solid mineral material, while the fine or smaller pollutants are less than 1 micron in size and are liquid droplets consisting of an aqueous solution of salts and organic matter. As a result, new standards may soon result, in which the determination of aerosol mass and composition will be a function of particle size.
Mass monitoring of two ranges of pollutant size cannot be adequately performed using present methods such as high volume filtering and gravimetric weighing because such methods are too slow, require manual operations and are incapable of continuous on-line operation. Further, measuring techniques employing geiger counters, scintillation detectors and gas flow proportional counters and the like are unsuitable because they do not have the high sensitivity or efficiency required to obtain accurate data in a short period of time. This latter requirement is important because it has been experimentally found that the mass concentration of pollutants in each size range varies considerably over relatively short periods of time and is markedly influenced by such factors as rain.